1. Field of the Invention
The present invention generally relates to switched driver circuit for generating a switched drive signal including a dithering signal.
2. Description of Related Art
Many electronic control module products make use of constant current output driver ICs. The purpose of these circuits is to provide a constant current through a system load of interest. In automotive applications, typical loads are: transmission pressure control solenoids, idle bypass air valves, etc. Constant current driver circuits are usually implemented using either “linear” or “switching” approaches. Linear constant current drivers are usually less expensive, but generate significant heat. Switching constant current driver can be more expensive, but minimize heat generation.
When using constant current drivers, another common feature is a superimposed “dither” frequency. The dither frequency is much less than primary switching frequency. The dither switching waveform is typically used to keep the solenoid pintle in constant motion thereby significantly reducing the effects of static friction. The two important characteristics of the dither waveform are the dither frequency and the dither amplitude. To be effective, the dither frequency must be in the normal response range of the particular solenoid and the dither amplitude must be of sufficient magnitude to move the solenoid pintle the desired amount.
A common circuit topology for a switching constant current driver is a hysteretic control approach. This topology works by switching the output drive transistor “on and off” while monitoring the actual load current. In a low side configuration, when the output current falls below the lower hysteresis threshold, the drive transistor is turned “on”. When the current rises above the upper hysteresis threshold, the drive transistor is turned “off”.
Therefore the primary switching frequency is determined primarily by the actual load impedance. Unfortunately, when the load impedance increases to a point where the primary switching frequency approaches the desired dither frequency, the effectiveness of the dither waveform can be greatly reduced due to a reduction in the dither amplitude.
In view of the above, it is apparent that there exists a need for an improved driver circuit.